This invention relates to vehicle tires, and more particularly to heavy duty truck tires specifically designed to be used for long haul trucking operations. The invention is particularly useful in reducing irregular wear of front axle truck tires to extend their useful life.
Irregular tire wear includes namely full shoulder wear, feather wear, shoulder cupping, rib depression, flat spotting wear, erratic depression wear, rail wear and more. A complete explanation of all types of irregular tire wear can be found in a guide entitled "Radial Tire Conditions Analysis Guide: A Comprehensive Review of Tread Wear and Conditions", published by The Truck Maintenance Council (TMC).
There are many factors that can lead to premature tire wear. The conditions the truck is running in as well as the type of load the truck is carrying both have an impact on tire wear. This invention concerns namely the front axle tires used in long haul applications. In these applications, the majority of the time the tire is going to be running in a straight line, where there is very little torque, if any, applied to the front axle tires. This is opposite to regional applications, e.g. with pick up and delivery vehicles, which run with a lot of turning and twisting, a lot of starts and stops. Normal stresses become as important as the longitudinal and lateral stresses in producing irregular tire wear. Whatever amount of irregular wear started on the tire in regional operations, this irregular wear is erased because of all the scrubbing and sideways movement of the tires in the regional application; which causes a rather high rate of wear. Abnormal wear is much less of an issue in regional operations simply because the dominant wear process is global scrubbing over the contact area of the tire tread.
Truck tires that experience straight ahead driving for a large portion of their life develop wear patterns that are unique. Tire stresses from turning and maneuvering the vehicle only exist for a limited time compared with tire stresses from straight ahead highway driving. The leading cause of irregular or premature tire wear is improper inflation pressure, with alignment coming in a close second. Poor alignment of the wheels is a common cause of irregular wear as well as compliance of the truck's suspension and steering linkages. Poor alignment include improper drive axle thrust angle, elasticity in the steering system, and drive axle lateral offset. Some of these factors are under the control of the owner and can be minimized with good maintenance practices. Others are inherently present within the steering and suspension system designs for state-of-the-art trucks.
In long haul application, irregular or uneven wear remains an issue because the tires, as they are improved in endurance, are able to last for an extremely high number of miles at a very slow rate of wear. Therefore, the chance that uneven or irregular wear governs the tread life of the tire increases. Uneven wear is a complex phenomenon that is very difficult to address. There are many different factors that can cause irregular tire wear, and it is most often a combination of them, rarely just one that will start uneven wear or cause it to increase. The combination is what makes it difficult for designers to find the right causes for an uneven wear problem. Long haul steer axle truck tires commonly have a tread pattern with a series of circumferential ribs separated by circumferential grooves. Irregular wear of rib type tires frequently starts at the edges of these ribs causing depressions, flat spots and the like which progress across the rib and along the rib with increased mileage. This is commonly called railway wear. By controlling the onset of this irregular wear pattern the user can increase the useful tread life of the tire's tread.
Although many factors seem to be closely related to maintenance parameters, the prior art already knows many attempts to try to improve the intrinsic ability of the tire to sustain long haul operations without developing uneven wear. Reference can be made to U.S. Pat. No. 4,840,210 that discloses an attempt to keep under control the shape of a shoulder area of a tire's tread in an effort to limit the onset of irregular changes in the tread as the tire wears. The solution proposed in the reference requires to mount the tire on the rim on a preselected way because of the asymmetrical profile. In U.S. Pat. No. 5,099,899 the railway wear is suppressed by increasing the zig-zag angle of the main circumferential grooves as the tread wears out. Yet another means to control irregular wear at the shoulders is disclosed in U.S. Pat. No. 4,214,618. A circumferentially extending narrow zig-zag groove at the edge of the shoulder contact surfaces reduces the railway wear and growth thereof toward the center of the tread.
A need remains to reduce the stresses on the main ribs of the steer axle truck tire during straight ahead driving. Although the prior art already knows several tires that have proved to work satisfactorily in the above mentioned long haul operations, a need exists to further improve the way the contact area of long haul truck tires can best comply with the supporting surface when subjected to wearing stresses. This improvement is best accomplished by a tread pattern which is designed to delay the onset of irregular wear for the long haul truck tire.
Accordingly, an object of the present invention is to reduce the onset of irregular wear of steer axle truck tires and reduce the growth thereof during long haul operations. The onset of railway wear that is adjacent to circumferential grooves is of particular concern.
Yet another object of the present invention is to construct a truck tire with a belt package, a carcass, bead areas and sidewalls that includes a tread design that assists the circumferential ribs to enhance the reduction in irregular wear in the main tread area of a tire.